The trunk on one of my about 15 yr old Plumerias has hard bark on it and even has some round sections flaking off past the stem. What caused this and what can I do? It still blooms every year

barbsbrigade
You definitely have had some damage. You don't say where you are located but I am thinking it could have been either sunburn or cold damage, although cold would have also damaged the tips. As long as there is a sufficient layer of cambium left the tree should be OK. If you live in a very dry hot climate you may want to protect your plant from sunburn during the hottest months.